Is spell range calculated like movement, such that diagonals cost 1.5 times the distance?
Yes, distance for spell use is calculated just like distance for movement.
There are templates in the rule book that show coverage by spell range on battlemats. Our group has these cut out into colored plastic translucent sheets so we can put them on the battlegrid to see exactly where the effect occurs.
You're talking about area of effect. I'm asking about range.
If a spell has a range of 30 ft. and the target is six squares away, he is in range. But what if he is five diagonal squares away? To move that distance would require a character to move 35 ft. If spells are calculated the same way, the target would be out of range.
It works just like it does for other things. Every other diagonal counts for 10'.
It's all the same Tarondor.